1. Field of the Invention
The invention relates to planar broad bandwidth devices for propagating microwave energy. More particularly the invention relates to symmetrical 180.degree. hybrids that exhibit nearly perfect amplitude and phase balance as well as very high isolation between the sum and the difference ports at all frequencies.
2. Description of the Prior Art
Microwave propagation devices are used on a vaiety of applications, including electronic counter measures, as well as in applications such as communications and stochastic cooling for particle accelerators. The requirements for microwave propagation devices differ from application to application. For example, in many communications applications, only a narrow band of frequencies are utilized and only narrow bandwidth devices are therefore necessary. In the other applications broad bandwidth devices are desirable to enable to use of signals having a wider range of frequencies.
Important aspects of the invention relate to the operation of microwave hybrid devices. Such devices are referred to by several terms in the literature including: Transverse Electromagnetic ("TEM") hybrids; 180.degree. hybrids; magic tees; 180.degree. couplers; TEM magic tees; and rat race hybrids. These devices will be generally referred to herein as 180.degree. hybrids, a 180.degree. degree hybrid being generally defined as being a four port device that for a particular frequency band exhibits certain properties to a specified accuracy. The four ports will be referred to throughout this specification as the sum port, the difference port, a first port and a second port. The characteristic properties are stated as follows:
1. A signal applied to the sum port will split to provide equal in-phase signals at the first and second ports with no signal at the difference port;
2. A signal applied to the difference port will split to provide equal signals 180.degree. out of phase at first and second ports with no signal at the sum port;
3. A first signal applied to the first port and a second signal applied to the second port will provide signals at the sum and difference ports that represent the sum of the first and second signals and the difference of the first and second signals respectively;
4. The first and second ports are isolated from one another.
In order to meet operational requirements, 180.degree. hybrids need to exhibit these characteristics. The range of frequencies over which the device exhibits the required characteristics to within acceptable bounds defines its operational band. If f.sub.1 is defined as the lower bound of the operational band and f.sub.2 as the upper bound then the width can be defined as f.sub.2 -f.sub.1 and the relative bandwidth as (f.sub.2 -f.sub.1)/f.sub.1. An octave bandwidth is achieved when f.sub.2 =10f.sub.1. The greater the operational band, the greater the usefulness of 180.degree. hybrids in broad band applications.
The operational bands of prior art 180.degree. hybrids vary with their construction. Consider one commonly used 180.degree. hybrid known as the common rat race or ring hybrid. The impedances at the two output ports of this device are only identical at one or two frequencies at best and the signals at the first and second ports when driven at the sum or difference port are only perfectly balanced at one or two frequencies. Modified ring hybrids have been created with improved perfermance. None, however, make the bandwidth larger than about half an octave.
A further 180.degree. hybrid not commercially available that is operational for bandwidths of about an octave and uses coupled slot lines is described by Aikawa and Ogawa in IEEE Trans. on MTT, Vol 28, No. 6 (June 1980). This type of device should exhibit nearly perfect amplitude and phase balance over octave bandwidths. the isolation between the sum and difference ports should also be quite high for these bandwidths. This type of device is not, however, operational over bandwidths greatly in excess of one octave.
In applications in which wider bandwidths and/or higher performance are required a 90.degree. hybrid followed by a Schiffman phase shifter is generally utilized to provide a 180.degree. hybrid. This type of device is most commonly constructed in strip line but it is also possible for it to be constructed in microstrip. For highest performance the strip line used in this type of device is constructed on a low permeability dielectric such as Teflon/glass and multiple stage offset couple strip lines are used to make the 90.degree. hybrid and the Schiffman phase shifter. In order to achieve good performance in this type of device over wide bandwidths, considerable precision in construction is required. The requisite degree of precision is difficult to achieve for very broad bandwidths and the construction results in the utilization of quite narrow metal foil lines. The narrow lines put limits on the amount of power that can be put through such devices. This type of device works quite well over octave bandwidths and although devices have been made with bandwidths of 2 octaves the manufacture of devices with broader bandwidths than these is restricted by the previously mentioned construction limitations.
The "Tapered Asymmetric Microstrip Magic Tee" described by M.H. Araim and N.W. Spencer in IEEE Trans. on MTT Vol. 23 No. 12(Dec 1975) describes a 180.degree. hybrid that can function over bandwidths in excess of a decade. The device is constructed using a cascade of two -8.39dB asymmetric microstrip couples. The asymmetric construction of the device means that it does not exhibit perfect amplitude and phase balance at all frequencies. The power handling capabilities of this device are also limited.